Light emitting diodes (LEDs) are often arranged in series and/or parallel combinations as lines/strings or arrays for particular lighting applications. An LED is electrically a diode which conducts in one direction only, just like diodes used for non-optical applications. LEDs are inherently low-voltage devices with a luminous output proportional to a forward drive current. Conventional LED lighting systems therefore include some sort of current driver, designed to convert available power such as AC power from the mains to a DC current suitable to drive LEDs. Drivers can be designed to drive single LEDs or to drive systems comprising a multiplicity of LEDs arranged in series and/or parallel. When driving a multiplicity of LEDs, a failure such as a short circuit or open circuit means any single LED can cause complete failure of the system by either failing to drive or damaging remaining LEDs.
There are also LED drivers that use AC current. U.S. Pat. No. 7,573,729 B2 to Elferich and Lurkens discloses a resonant circuit located on the primary side of an output transformer; the secondary side drives LEDs, paired with reversed polarities so that one LED of each pair conducts during each half cycle of the AC current. Multiple pairs can be connected in series. However, this design is also sensitive to failure of individual LEDs. A string of many pairs looks like a single element to the drive circuit, and a failure of any component within the string can cause the entire string to be disabled. Further the required resonance can be destroyed.
U.S. Pat. No. 8,145,905 B2 to Miskin et al. discloses another driver using AC current and “anti-parallel” LEDs. Miskin discloses a “fixed high frequency inverter” having a fixed frequency and voltage AC output. The inverter drives various possible networks of LED couplets (the anti-parallel LEDs). Current can be adjusted to individual couplets or series strings of couplets using a capacitor or resistor. No series or parallel inductor is used in the LED circuit and no bypass capacitors are used. The output circuit is driven at a specific frequency and specific voltage and does not take advantage of any inherent resonance. The resulting system is sensitive to failure of single LEDs. The current waveforms in the LEDs are likely to exhibit significant harmonic distortion and are therefore likely to emit significant radio frequency interference. Overall energy efficiency is not as high as in a resonant system.
U.S. Pat. No. 6,826,059 B2 to Böckle and Hein discloses an LED driver based on ballasts for fluorescent lighting. The output is a resonant circuit. The LEDs are configured in strings or arrays, with either one array or two arrays arranged in opposite polarity. Each array consists entirely of LEDs with no reactive components. A single inductor and two capacitors outside of the arrays complete the resonant circuit; there are no reactive components distributed through the LED arrays.
What is needed is a drive circuit that can self-adjust to provide controlled power to individual elements in an LED array that is additionally insensitive to the failure of individual LEDs (short circuit or open circuit) and does not require additional active semiconductor components.